In order to improve the fuel efficiency and to reduce CO2/NOx, research and development of automobiles and the like having an internal-combustion engine have been conducted. In recent years, for this purpose, a combustion pressure sensor to be mounted on an internal-combustion engine has been attracting attention. Since a combustion pressure sensor directly detects the torque fluctuation, both of the low torque fluctuation and the low CO2/NOx can be achieved, therefore, the improvement of the fuel efficiency and the reduction of CO2/NOx are expected.
Such a combustion pressure sensor is constituted of a piezoelectric material, and as the piezoelectric material, La3Ga5SiO14 (langasite), a langasite-type crystal having the same crystal structure as the crystal structure of langasite, and the like are known.
Langasite, and a langasite-type crystal do not generate the phase transition until reaching the melting point, and do not have pyroelectricity, therefore, are characterized in that characteristic deterioration at high temperature is small, and there is no fear of electrical signal disturbance such as generation of electromotive force corresponding to the temperature change due to pyroelectricity and of dielectric breakdown due to pyroelectric voltage. However, even in these langasite and langasite-type crystal, the electric resistivity at high temperature is not sufficient for use in an automotive combustion pressure sensor.
Recently, it has been reported that among the langasite-type crystals, an order-type langasite-type crystal having an ordered structure in which each element is regularly arranged at atomic sites in the crystal is preferred for high temperature sensing application (for example, see Non-Patent Literature 1). According to Table 1 of Non-Patent Literature 1, it is shown that the electric resistivities at 500° C. of Ca3TaGa3Si2O14 (CTGS) and Ca3TaAl3Si2O14 (CTAS) are 1.7×109 Ω·cm and 2.7×109 Ω·cm, respectively, and according to FIG. 1, it is shown that the electric resistivity at 427° C. of CTGS is 1.0×1010 Ω·cm.
However, even in CTGS and CTAS in Non-Patent Literature 1, it cannot be said that the electric resistivity at high temperature is sufficient.
In addition, focusing on the La3Ta0.5Ga55O14 (LTG) crystal that is a langasite-type crystal, a crystal of Ca3TaGa3Si2O14 (CTGS) has been designed on the basis of this electric conduction mechanism (for example, Non-Patent Literature 2). According to Non-Patent Literature 2, it has been disclosed that in LTG, the electric conductivity improves due to the metal-deficiency defect, therefore, a CTGS crystal in which the Ga/Ta ratio and the Si content are smaller than those of the stoichiometric composition so as to accompany anti-site defects and to minimize the occurrence of metal deficiency defects has an electric resistivity of 1.8×1010 Ω·cm at 400° C.
However, according to Non-Patent Literature 2, among the langasite-type crystals, only CTGS has been disclosed, and CTAS has not been developed yet. In addition, it cannot be said that CTGS of Non-Patent Literature 2 also has sufficient electric resistivity at high temperature.